This invention relates generally to improved methods for efficient purification of gases, and more specifically, to methods for both effectively purifying air and simultaneously destroying most potentially harmful airborne chemicals, microorganisms and other biologicals by both chemical and electrochemical means.
Modern society is facing a major problem of increased pollution of the air we breathe in the work environment, homes, hospitals, public buildings and vehicles, caused by a variety of toxic, volatile, and often malodorous and irritating chemicals, as well as potentially dangerous microorganisms, such as viruses, bacteria, molds, yeasts, spores and other pathogens. In the industrial environment, toxic, hazardous, or contaminating chemical vapors, such as formaldehyde, benzene, chloroform, etc., have come under strict government regulation; however, instantaneous worker exposure can be high in case of a spill and often there is little or no provision for rapid removal of such substances from the air to an acceptably low level. Moreover, there often is an accumulation of these airborne pollutants in the vicinity of chemical plants, food processing operations, sewage treatment plants and utilities. Significant or even larger contaminant or pollutant levels can occur indoors because of tightly enclosed, more energy efficient buildings. For hospitals, the continued use of ethylene oxide as a disinfectant has created concern, but an even more immediate dange is the large number of annual mortalities caused by hospital infections. Likewise, air-conditioning units in large buildings have been associated with deadly "Legionnaires Disease".
Kirk-Othmer, 3rd. Edition, Vol. 1, p 653 (Wiley Interscience) lists a variety of methods for removal of pollutants in air, including adsorption, e.g. in activated carbons, absorption in solution, filtration, coagulation, electrostatic precipitation, incineration, chemical reaction, condensation, etc. However, many of these methods cannot readily remove very tiny particles, such as gas molecules and smaller microorganisms.
Absorption of toxic gases by water or aqueous solution, such as HCl, HF, NH.sub.3, Cl.sub.2, H.sub.2 S, amines, etc., can be very effective. Likewise, many of these gases, as well as microorganisms, can be removed effectively by adsorption onto solid surfaces, particularly activated carbon. Absorption, adsorption, filtration, coagulation and electrostatic precipitation are physical methods of decontamination. Physical methods are of special interest where recovery of chemicals is desirable economically.
Chemical methods of decontamination include reactions of toxic or hazardous substances with such oxidizing agents as chlorine, chlorine dioxide, hypochlorite, ozone, peroxide or reducing agents such as dithionite, noble metal catalysts and hydrogen, metallic sodium, etc.
Both physical and chemical methods have certain drawbacks however. For example, in situations where it is not desirable to recover airborne chemicals for resuse, the problems of disposal still exists. Also, physical techniques suffer from "saturation" related dangers. For example, in a hospital or "clean-room" situation, carbon-based filters or cartridges can become saturated to the point where no further removal occurs or where dangerous desorption takes place. In contrast, chemical methods often require use of reagents which are themselves quite toxic and often have problems of byproduct disposal. Other methods, such as incineration can be uneconomical or even illegal in certain areas.
In an effort to overcome some of the shortcomings previously noted, electrochemical methods have been tried. For example, U.S. Pat. No. 3,725,226 (Stoner) describes an electrochemical device with graphite electrodes. Pathogens in water are destroyed by periodically reversing DC current. Stoner fails to address the problems associated with air purification, including means for removing harmful substances from air like organic chemicals and their destruction. French Patent No. 1,538,901 (Marzluff et al) teach air purification means capable of destroying a wide range of volatile chemicals, including aldehydes, alcohols, esters and others. Metal oxide anodes, such as lead dioxide and porous air cathodes containing carbon are employed in an electrochemical cell separated by glass wool impregnated with sulfuric acid. Both the Stoner and Marzluff et al patents fail to disclose an effective method for scrubbing toxic airborne substances from air. Moreover, Marzluff et al's electrochemical cell requires the airborne components contact the electrode before destruction of the pollutant can occur placing a severe limitation on mass transport on their apparatus and rate of degradation, especially for contaminants present in air at lower, but still dangerous levels of concentration. Marzluff et al do not utilize a regeneratable electrolyte for further chemical reaction with pollutants.
Methods have also been developed for the removal of toxic and environmentally unacceptable emissions from utilities and from sour gas, usually with the objective of making a useful chemical, instead of forming an essentially innocuous by-product. For instance, U.S. Pat. No. 4,426,364 (Cooper) discloses a process for removing nitrogen oxides and SO.sub.2 from gas mixtures including air, by contacting the gas mixture in a scrubber with an aqueous solution of an acid and an oxidizing agent, such as peroxide or persulfate in which nitric and sulfuric acids are formed for recovery. U.S. Pat. No. 4,643,886 (Chang et al) discloses a process for removal of H.sub.2 S from sour gas, which is largely methane, comprising contacting the sour gas stream with an aqueous alkaline solution at a temperature below the melting point of the product, namely sulfur. The aqueous solution comprises at least one polyvalent metal chelate in a higher valence state in an effective amount suitable for oxidizing all the H.sub.2 S to recoverable sulfur. The inactive form of the polyvalent metal chelate is regenerated anodically in an electrochemical cell, and the solution recycled to the contact zone. The process of Chang et al is conducted under conditions which favor the formation of elemental sulfur for recovery, instead of minimizing or avoiding the production of by-products requiring separation, purification or disposal, or which are consumed by the process, in-situ.
A further representative example of an electrochemically based system is that from Pacific Engineering & Production Co. (Henderson, Nev.) under the trademark, Odormaster. This apparatus, used to eliminate odors in sewage and industrial plants, utilizes an electrochemically generated aqueous sodium hypochlorite solution which circulates through a scrubber. This technology removes the mass transport limitation noted above with the Marzluff et al process, by providing a large reservoir of an oxidizing agent to destroy malodorous components. However, a serious limitation exists in the Pacific Engineering approach in that many kinds of organic compounds, such as olefins and aromatic hydrocarbons will be chlorinated should they enter the electrochemical cell. As a general rule, chlorinated hydrocarbons are more toxic than the parent hydrocarbon, and furthermore, are usually more difficult to destroy by oxidation, as for example, polychlorinated biphenyls.
U.S. Pat. Nos. 3,975,246 and 4,048,044 (Eibl et al) disclose electrochemical means for purifying water contaminated with microorganisms. In the later patent, water is disinfected by an oxidation process at the anode without disinfectant additives. The former patent also relies on anodic disinfection, but also requires chloride, hydroxide, carbonate, etc., in the cathode compartment in concentrations exceeding those in the treated water. Both Eibl et al patents rely principally on electrochemical means for disinfection, and consequently, the scope of contaminants and toxic substances which can be destroyed is limited.
Accordingly, it would be desirable to have an improved method for purifying air and other gases, including means for separating and destroying a broader range of potentially toxic airborne chemicals and microorganisms recovered therefrom, by combining both chemical and electrochemical methods.